By analyzing the change of an index for the characteristics of South Asia High and variations of upper-air troughs in 2002-2005,we studied the impact of South Asia high on the beginning and ending of the Mei-yu (i.e.sustained rain corresponding to the ripening season of plum) in Jiangsu province.Statistic verification is conducted on the relationships between the index and the Mei-yu season in 1991-2005 to examine the impacts of the SAH characteristics index on a rain intensity index of Mei-yu and regional distribution of a characteristics index for different annual patterns of Mei-yu.Historical composite is performed of the 100-hPa circulation field for these patterns using the 100-hPa geopotential height of Northern Hemisphere from 2002 to 2005 and 45-year NCEP reanalysis to study the difference in the circulation for different patterns of Mei-yu.Diagnostic and statistic conclusions,which share much in common,have been obtained as follows.(1) The characteristics preceding to and the advancement/retreat of SAH and the movement of westerly troughs are the factors that influence the onset time of the Mei-yu season; after the Mei-yu onset,the progression/withdrawal of SAH and how farther east it extends are determining how long the Mei-yu lasts and when it ends.(2) During the Mei-yu,the general 100-hPa circulation situation and average characteristics of the SAH are well corresponding to the characteristics of the season and annual patterns of Mei-yu.In addition,the averages of the SAH ridgeline and east-extending index for June,July and the Mei-yu season have some implications to the forecast of the index of Mei-yu intensity.These conclusions can be served as powerful means in determining the starting/ending dates,duration and annual pattern of the Mei-yu season.